Wet elecrostatic precipitator

ABSTRACT

(E( A DISCHARGE ELECTRODE STRUCTURE SUPPORTED WITHIN SAID PASSAGE, (F) MEANS TO APPLY A HIGH VOLTAGE BETWEEN SAID DISCHARGE ELECTRODE STRUCTURE AND BOTH OF SAID TUBES TO CAUSE MIGRATION OF PARTICLES IN SAID GAS TOWARD THE FILMS ON SAID TUBES AND THEREBY PRODUCE A CLEAN GAS, AND (G) OUTLET MEANS AT THE UPPER END OF SAID ANNULAR PASSAGE TO DISCHARGE THE CLEAN GAS INTO THE ATMOSPHERE, SAID OUTLET MEANS INCLUDING AN INVERTED FRUSTO-CONCIAL DEFLECTOR IN AXIAL ALIGNMENT WITH SAID TUBES TO DIRECT THE CLEAN GAS FROM SAID PASSAGE OUTWARDLY IN THE HORIZONTAL PLANE. 1. AN ELECTROSTATIC PRECIPITATOR FOR CLEANING CONTAMINATED GAS COMPRISING: (A) CONCENTRICALLY-ARRANGED INNER AND OUTER COLLECTOR TUBES DEFINING A VERTICALLY-DISPOSED ANNULAR GAS PASSAGE, (B) MEANS COUPLED TO A WELL TO DRAW LIQUID THEREFROM AND TO FEED THE LIQUID TO THE UPPER ENDS OF SAID TUBES TO PRODUCE A DOWNWARDLY-FLOWING AND SUBSTANTIALLY UNIFORM LIQUID FILM ON THOSE SURFACES OF SAID INNER AND OUTER TUBES WHICH LINE SAID PASSAGE, (C) CONCENTRICALLY-ARRANGED TROUGHS AT THE LOWER ENDS OF SAID TUBES TO RECEIVE THE DOWNWARDLY-FLOWING LIQUID THEREFROM AND TO DISCHARGE THE LIQUID INTO SAID WELL, (D) INLET MEANS TO INTRODUCE SAID CONTAMINATED GAS INTO THE LOWER END SAID ANNULAR PASSAGE BETWEEN SAID TROUGHS, SAID INLET MEANS INCLUDING MEANS TO ADMIT SAID GAS IN A HORIZONTAL PLANE AND FURTHER INCLUDING A FRUSTO-CONICAL DEFLECTOR IN AXIAL ALIGNMENT WITH SAID TUBES TO DIRECT CONTAMINATED GAS ENTERING IN THE HORIZONTAL PLANE UPWARDLY THROUGH THE VERTICALLY-DISPOSED ANNULAR PASSAGE,

Feb. 20, 1973 A. P. DE SEVERSKY 3,716,966

WET ELECTROSTAT I C PREC IP I TATOR Filed Aug. 31, 1960 5 Sheets-Sheet 1INVENTOR. Q y Aumq Horseman Arrows Feb. 20, 1973 A. P. DE SEVERSKY3,716,

WET ELECTROSTATIC PRECIPITATOR Filed' Aug. 31, 1960 5 Sheets-Sheet 2 INWIN TOR.

BY Awumq P 0: Jumps" nr' wers Feb. 20, 1973 A. P. DE S'EVERSKY 9 6 WETELECTROSTATI C PR EC I PITATOR Filed Aug. 31, 1960 r 5 Sheets-Sheet I5vIN V EN TOR. Acumq 0! Sue Arm Feb. 20, 1973 A. P. DE SEVERSKY 3,715,966

WET ELECTROSTATIC PRECIPITATOR Filed Aug. 31, 1960 5 Sheets-Sheet 4 Hus:Gene-49AM Ti 1U. Mum

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6o canoe P 0: Screw a /W'W U.S. Cl. 55-118 4 Claims This inventionrelates generally to apparatus for clean ing contaminated gases and moreparticularly to a wet electrostatic precipitator of exceptionalefficiency.

This application is a continuation-in-part of my copending applicationSer. No. 855,369, filed Nov. 25, 1959, and issued Sept. 11, 1962 as Pat.3,053,029, which copending application is a division of my originalapplication Ser. No. 479,909, filed Jan. 5, 1955, and issued as Pat.2,937,709 011 May 24, 1960.

vThe increase in atmospheric pollution and smog in modern communitieshas created a health hazard of major proportions and has become a matterof grave social concern. Air pollution is irnputable to many factorsamong which are the use of incinerators to burn household waste andvarious industrial operations which discharge combustion products intothe atmosphere.

Electrostatic precipitators, both of the dry and wet type, have beenused for cleaning contaminated gases, but such use has been limitedmainly to industrial applications. There has not heretofore beenavailable an inexpensive, efiicient and reliable precipitator fornon-industrial users, to be installed for example in apartment houses soas to prevent the discharge into the atmosphere of particles emanatingfrom incinerator and heating systems.

The present invention deals with a precipitator of the wet type whereinthe contaminated gases are conveyed through an electrostatic fieldbetween electrode surfaces. Particles in the gas are precipitated onto acollecting surface constituted by a film of water flowing over acollector. Since the water carries the particles away continuously, aprecipitator of this type is self-cleaning and is therefore particularlysuited to non-industrial uses. This wet type of precipitator is alsoadvantageously used for extracting radio-active particles from theatmosphere in case of fallout. The dry type of precipitator wouldaccumulate the extracted particles and become so highly radioactive thatit would become a hazard itself. By using the wet type of precipitator,the radioactive material is carried away by the liquid which may then bestored or treated to decontaminate. While the present invention will bedescribed in connection with a wet precipitator, it will become apparentthat certain features of the invention are also applicable to dryprecipitators.

.One of the characteristic defects in existing wet precipitators arisesfrom the fact that the liquid film is generally uneven and does notfully coat the collector surface. If the water is merely poured into thevertical collector tube of the precipitator, it tends to trickle down inseparate streams and it is difficult to ensure that it will spread overthe interior surface without an excessive flow of incoming water. As aresult, dry patches appear on the collector and certain particles, suchas carbon black, on reaching a dry area tend to give up theircharge.'These particles acquire an opposite charge by induction and mayexperience sufficient force in a strong field to be moved back towardthe other electrode into the gas stream. As a consequence, the particlespass out of the cleaner and are discharged into the atmosphere.

Accordingly, it is one object of the invention to provide anelectrostatic precipitator of the water film type wherein water iscaused to flow uniformly and smoothly on the walls of the collectortubes, and wherein dry patches are obviated. A significant feature ofthe inven- United States Patent tion resides in the fact that a Waterdistributor is provided which discharges multiple streams tangentiallyagainst the surface of the collector, the streams being divergent andintersecting.

Another factor which militates against the use of wet precipitators innon-industrial applications is the relatively high power requirementsfor the precipitator. It is the usual practice to pass the contaminatedgas initially through a charging field in which a corona discharge isproduced in order to ionize the suspended particles. The gas then passesthrough an electrostatic precipitating field which is free of coronadischarge, this field acting to cause migration of the particles towardthe collecting electrode. The use of a pre-ionization field involves arelatively high voltage and in conventional precipitators brings about asubstantial current flow. The power requirements of the pre-ionizationfield when added to that of the precipitating field are considerable andmake the operation of the standard precipitator costly.

It is therefore another object of the invention to provide a wetelectrostatic precipitator of high efficiency which re quiressubstantially less power to operate than conventional gas cleaners.

A further object of the invention is to provide a power supply for aprecipitator which generates pulsed voltages of high amplitude toproduce an ionizing field.

Also an object of the invention is to provide a wet electrostaticprecipitator which is of compact design and yet has a large gas cleaningcapacity.

The wet precipitator in accordance with the invention is constituted bytwo concentric collector tubes, a water film being formed uniformly bothon the outer surface of the inner tube and the inner surface of theouter tube by means of distributors producing multiple diverging streamsof water which are introduced tangentially to the collector surface andintersect thereon. Precipitator and corona discharge electrodes aresuspended in the annular passage between the two tubes to removeparticles from. the gas conveyed vertically thereon. Corona dischargevoltages are applied in pulsatory form, whereas the lower precipitationvoltage is maintained at a constant level.

For a better understanding of the invention as well as other featuresand objects thereof, reference is made to the following detaileddescription to be read in conjunction with the accompanying drawingwherein like components in the several views are identified by likereference numerals.

In the drawings:

FIG. 1 is a perspective view of the precipitator cabinet.

FIG. 2 is a section taken through the precipitator structure in thevertical plane.

FIG. 3 is a transverse section taken in the plane indicated by line 3-3in FIG. 2.

FIG. 4 is a linear projection of the flange water distributor.

FIG. 5 is a plan view of the flange.

FIG. 6 is an end view of the flange.

FIG. 7 shows the water distribution pattern produced by the flange.

FIG. 8 schematically shows one preferred form of preionization andcollector electrode structure in accordance with the invention.

FIG. 9 schematically shows another form of pre-ionization and collectorelectrode structure in accordance with the invention.

FIG. 10 shows the waveform of voltage applied to the structure shown inFIG. 9.

FIG. 1 1 is a schematic view of a pulsating high voltage supply suitablefor use in conjunction with the present invention.

FIG. 12 is a sectional view of a preferred form of a portion of the twoshells forming the collector assembly.

Referring now to the drawings and more particularly to FIGS. 1 to 3, apreferred embodiment of the wet precipitator in accordance with theinvention is shown housed in a metal cabinet constituted by side panels10, a base plate 1 1 and a cover 12. The water supply for theprecipitator is contained in a sliding tray 13 disposed at the bottom ofthe cabinet, and serving as a well. Mounted in the tray is a water pump14, which may be electrically driven.

Vertically mounted within the cabinet is a collector assembly includinga cylindrical inner collector shell 15. Concentrically disposed aboutthe inner shell is an outer collector shell 16, an annular air passage17 being defined between the shells. The outer shell is supported withinthe cabinet by means of a mounting ring 19.

Supported above the collector assembly at the top of the cabinet is afan motor 20, the motor being positioned centrally within an invertedconical deflector 21. Positioned in the space between the deflector 21and the collector assembly is a fan 22, the fan being attached to theshaft of motor 20. Air drawn by the fan 22 through the air passage 17 isexhausted through a set of louvered grills 23 mounted on the panels ofthe cabinet. Air enters the cabinet through a set of intake grills 24,the air being deflected upwardly into the air passage 17 by means of atruncated conical deflector 25.

Water sucked from tray 13 by the pump 14 is supplied through a flexibletube 26 to a water distributor 27 mounted atop the inner collector shell15, the upper end of the shell being closed by a sealing disc 28. Waterfrom the distributor is supplied through radially extending pipes 29 toan outer water ring 30 lying above the mounting ring 19 and providedwith passages 31 which conduct the water to distribution channels formedin a circular flange 32. Flange 32 lies against the inner surface of theouter shell 16 and acts to supply water therein in a manner producingthereon an evenly distributed film of downwardly flowing water.

Water from the distributor 27 is also fed to a water ring 33 connectedto the upper end of the inner shell 15 and having a passage 34 thereinwhich supplies the water to the distribution channels formed in acircular flange 35 which lies against the outer surface of the innershell 15 in a manner also producing an evenly distributed film ofdownwardly flowing water.

The structure of the water distribution flanges can best be seen inFIGS. 4, and 6 which show the circular flange 35 for the inner shell ina straight line projection. It will be seen that formed on the surfaceof the flange is an array of equi-spaced tapered ribs 36 which are allinclined relative to the vertical to define a series of diverging orflared water channels '37. Water enters the flange from the associatedring at the channel entrances 37a and is discharged against the surfaceof the collector shell at the channel exits 37b.

As best seen diagrammatically in FIG. 7, the flow of water through thedivergent channels 37 causes the water at the exits thereof to fanoutwardly. The diverging patterns from the various channels intersect ata point slightly beyond the exits and the resulting interferenceprevents build-up of the film and tends to produce an even waterdistribution along the collector surface. This action is further aidedby the whirling motion imparted to the water by reason of the incline ofthe flange channels, the water thereby entering tangentially andspirally along the collector surface. The combined action of thediverging multiple streams of water and the whirling motion of thestream has been found to result in a uniform water film which adheres tothe surface of the collector and avoids the presence of dry spotsthereon. The flanges may in practice be made of rubber, nylon or similarmaterial.

Attached to the lower end of inner collector shell 15 is a base ring 36and attached thereto is a circular gutter 38A, the gutter beinginterposed between the upper end of conical deflector 25 and the lowerend of the inner shell 15 so as to receive the downwardly flowing watertherefrom. Positioned at lower end of shell 16 is an annular trough 38,the inner wall of the trough extending above and being spaced from thelower end of the outer shell 16 to receive the water flowing downwardlythereon. Suitable pipes 18 are provided (not shown) to return the waterreceived in gutter 37 and trough 38 to the tray well 13 forrecirculation. Thus continuous water films are produced in bothcollector shells.

Supported above the water distributor 27 and insulated therefrom is aconductive spider ring 39 from whose radially extending horizontal arms40 are vertically suspended a group of wire precipitator or dischargeelectrodes 41, the wire electrodes lying midway between the inner andouter shell in the air passage 17. Pre-ionization electrodes (not shown)may be attached to the lower end of the precipitator electrodes. Asuitable high-voltage power supply 42 may be housed within the spaceprovided within the inner shell, or it may be mounted adjacent the motor20at the top of the cabinet.

The high voltage supply is constituted by any known high voltagegenerator whose negative terminal is connected to the water pool andwhose positive terminal is connected to the precipitator electrodes. Asthe water is pumped continually by the pump and allowed to falluniformly down the collector surfaces, the potential applied to the poolby the supply will also be applied to the water films or curtains on thecollectors. Thus an electrostatic field is established between theelectrodes and the two fluid collector surfaces on the walls definingthe annular passage 15.

The precipitator shown is capable of cleaning the air in a largeenclosure, the air being drawn in through the intake grills and beingconveyed through the air passage between the collector electrodes, Watersucked up from the pool is caused to flow in a tubular curtain down thecollector shells, the contaminating particles being precipitated andwashed down the collectors and being received in the pool. Suitablefilters are provided in conjunction with the pump to prevent the returnof the particles to the collectors. The air is expelled horizontally andomnidirectionally through the exit grills. It will be obvious thatessentially the same system may be used for any cleaning application andthe intake may be any form of contaminated gas.

In the electrode system shown in the above-noted figures, theprecipitator electrodes may be provided with discharge needles or pointsto effect pre-ionization of the gas entering the passage. The electrodesare of relatively large diameter whereas the needles are of smallerdiameter and have sharp edges to provide a sufliciently high voltagegradient to cause ionization and corona discharge. The corona dischargeacts to ionize the particles, such that when they enter theelectrostatic field they will be caused to migrate toward thecollectors.

To eifect power economy, there is provided an electrode arrangement asshown in FIG. 8, wherein precipitator electrodes 50 supported from aring 51 are connected to a source of direct-voltage 52 which generates aconstant voltage, say in the order of 15,000 volts. Suspended below theelectrodes 50 by a ring 53, and insulated from the electrodes 50, arecircumferentiallyarranged corona producing discharge points 54 to whichare applied periodic pulses having a magnitude in the order of 30,000volts. The periodicity of the pulses may be in the audio range orhigher.

Since the duty cycle of the pulses is relatively brief, the integratedamount of current drawn is fairly low despite the high voltage. Thus theparticles in the air entering the structure are first ionized by thehigh potential pulses and the ionized particles are then caused tomigrate by the precipitator electrodes which need not operate at so higha potential. For purposes of ionization, a constant voltage is notessential since onceionization occurs no useful purpose is served by theionization field.

The pulse generator 55 may be of any known design,

such as is used in radar techniques, and may include differentiatingnetworks or other pulse shaping means.

An alternative electrode structure is shown in FIG. 9, wherein thedischarge electrode for creating corona effects is constituted by aseries of rings 56 of relatively fine wire which girdles the group ofprecipitator electrodes 50. In practice, the electrodes 50 may be of ainch diameter and the rings of much finer diameter, such as 8 mil wire.The corona producing discharge points em ployed in the structure of FIG.8 may be used in conjunction with the wire rings for increasedeffectiveness. In this case, the same voltage is applied to both sets ofelectrodes and for this purpose a DJC. supply 57 is provided which ismodulated by a modulator 58 to provide pulses superimposed over aconstant voltage of the DC. supply. Thus, as shown in FIG. 10, thepulses 59 are developed above the DC. level 60, the pulse peak beingtwice the DC. level.

In the embodiment shown in FIG. 9, the number of rings used and thespacing thereof are dependent on the velocity of the gas stream and thefrequency of the high voltage pulses in the power supply. Thus, as thefrequency of the high voltage pulses increases, the number of rings maybe decreased, one ring being sufficient at frequencies of the order of1000 cycles and greater. Increasing the gas handling capacity of theprecipitator by increasing the velocity of the gases passingtherethrough will necessitate using a larger number of rings to ensurethe ionization of substantially all of the particles carried by the gasstream.

FIG. 11 depicts a schematic diagram of a high voltage pulse generatorsuitable for use with the precipitators herein described. High voltagesupply 61 is employed to produce two D.C. voltages of differentmagnitudes. Supply 61 embodies a quadrupler 62 and a doubler 63 producedoutputs which are related by a factor of two.

The output of doubler 62 is connected to curved contact 64. The outputof quadrupler 63 is connected to curved contact 65. The inner surfacesof contacts 64 and 65 lie along an imaginary circle whose center is atpoint 66. Wiper 67 is rotated about point 66 by variable speed motor 68.Wiper 67 may be connected by lead wire 69 to an electrical deviceutilizing pulsating high voltage, such as for example the electrostaticprecipitators of this invention.

The rotation of wiper 67 produces essentially a square wave voltagewhich fluctuates between the level of the output of the doubler 62 andthe level of the output of the quadrupler 63. It is necessary to confinecontacts 64 and 65 and wiper 67 in such a manner as to prevent sparking.This may be accomplished by submerging the parts in a high dielectricliquid, or, alternatively by enclosing the parts in a chamber which maybe maintained at a high gas pressure.

The frequency of the output voltage is controlled by the speed ofrotation of wiper 67. The duty cycle is fixed by appropriately choosingthe relative circumferential lengths of contacts 64 and 65.

Other pulsating high voltage systems may also be employed in conjunctionwith the electrostatic precipitators of the present invention. Thus forexample, instead of the contact and wiper arrangement shown in FIG. 11,either a mechanical chopper or an electronic control utillizing gastubes as electronic gate selectors are suitable.

FIG. 12 shows a sectional view of a portion of inner collector shell 15and outer collector shell 16. In the preferred embodiment shown in FIG.12 the outer surface of shell 15 and the inner surface of shell 16 arecoated with a porous ceramic, such as, for example, alumina. It has beendetermined that use of such a porous coating 70 facilitates theformation of a thin, uniform liquid film which is essential inorder toachieve high efficiency.

While there has been shown what are considered to be embodiments of theinvention, it will be manifest that many changes and modifications maybe made therein without departing from the essential spirit of theinvention. It is intended, therefore, in the annexed claims to cover allsuch changes and modifications as fall within the true scope of theinvention.

What is claimed is:

1. An electrostatic precipitator for cleaning contaminated gascomprising:

(a) concentrically-arranged inner and outer collector tubes defining avertically-disposed annular gas passage,

(b) means coupled to a well to draw liquid therefrom and to feed theliquid to the upper ends of said tubes to produce a downwardly-flowingand substantially uniform liquid film on those surfaces of said innerand outer tubes which line said passage,

(0) concentrically-arranged troughs at the lower ends of said tubes toreceive the downwardly-flowing liquid therefrom and to discharge theliquid into said well,

(d) inlet means to introduce said contaminated gas into the lower end ofsaid annular passage between said troughs, said inlet means includingmeans to admit said gas in a horizontal plane and further including afrusto-conical deflector in axial alignment with said tubes to directcontaminated gas entering in the horizontal plane upwardly through thevertically-disposed annular passage,

(e) a discharge electrode structure supported within said passage,

(f) means to apply a high voltage between said dis charge electrodestructure and both of said tubes to cause migration of particles in saidgas toward the films on said tubes and thereby produce a clean gas, and

(g) outlet means at the upper end of said annular passage to dischargethe clean gas into the atmosphere, said outlet means including aninverted frusto-conical deflector'in axial alignment with said tubes todirect the clean gas from said passage outwardly in the horizontalplane.

'2. An electrostatic precipitator for cleaning contaminated gascomprising:

(a) concentrically-arranged inner and outer collector tubes defining avertically-disposed annular gas passage,

(b) means coupled to a well to draw liquid therefrom and to feed theliquid to the upper ends of said tubes to produce a downwardly-flowingand substantially uniform liquid film on those surfaces of said innerand outer tubes which line said passage,

(0) concentrically-arranged troughs at the lower ends of said tubes toreceive the downwardly-flowing liquid therefrom and to discharge theliquid into said well,

(d) inlet means to introduce said contaminated gas into the lower end ofsaid annular passage between said troughs,

(e) a discharge electrode structure supported within said passage,

(f) means to apply a high voltage between said discharge electrodestructure and both of said tubes to cause migration of particles in saidgas toward the films on said tubes and thereby produce a clean gas,

(g) outlet means at the upper end of said annular passage to dischargethe clean gas into the atmosphere, and

(h) a high-voltage supply disposed within said inner tube to producesaid high voltage.

3. An electrostatic precipitator for cleaning contaminated gascomprising:

(a) concentrically-arranged inner and outer collector tubes defining avertically-disposed annular gas passage,

(b) means coupled to a well to draw liquid therefrom and to feed theliquid to the upper ends of said tubes to produce a downwardly-flowingand substantially uniform liquid film on those surfaces of said inner 78 and outer tubes which line said passage, said means 1,541,678 6/1925Bauer 55-155 to produce a downwardly-flowing liquid film on said2,233,639 3/1941 Pegg 55-118 inner and outer tubes including adistributor flange 2,631,684 3/ 1953 Schmidt 55-119 coupled to the upperend of each tube and provided 1,377,363 5/ 1921 Moon 55-10 withdiverging water channels inclined to direct water 5 1,425,637 8/ 1922Eschholz 55-137 in a plurality of diverging streams tangentially against2,504,858 4/ 1950 Mackenzie 55-138 X the tube surface, which streamsintersect on the sur- 2,529,045 11/1950 Ortgies 261-112 X face to formsaid substantially uniform water film 2,696,273 12/1954 Wintermute55-127 thereon, 1,263,941 4/1918 Schmidt 55-150 X (c)concentrically-arranged troughs at the lower ends 10 2,119,297 5/ 1938Scott 55-6 of said tubes to receive the downwardly-flowing liquid2,192,249 3/1940 White 55-13 therefrom and to discharge the liquid intosaid Well, 2,489,786 11/ 1949 Klemperer 55-138 (d) inlet means tointroduce said contaminated gas into I 2,864,458 12/ 195 8 De Graaf etal 55-118 X the lower end of said annular passage between said 3,026,9643/ 1962 Penney 55-138 X troughs, 15 2,926,749 3/ 1960 Oswald 55-108 X(e) a discharge electrode structure supported within 2,863,521 12/ 1958Davis 261-112 X said passage, 2,896,927 7/ 1959 Nagle et al 261-112 X(if) means to apply a high voltage between said dis- 2,971,453 2/ 1961Kinler 261-112 X charge electrode structure and both of said tubes to977,335 11/ 1910 Shaifner 55-139 X cause migration of particles in saidgas toward the 1,250,088 12/ 1917 Burns 55-118 X films on said tubes andthereby produce a clean gas, 1,357,202 10/ 1920 Nesbit 55-154 X and1,905,993 4/1933 Buff 55-118 (g) outlet means at the upper end of saidannular pas- 2,448,046 8/1948 Penney et a1. 55-118 sage to discharge theclean gas into the atmosphere. 2,615,530 10/ 1952 Hodson et a1 55-122 4.An electrostatic precipitator as set forth in claim 3, 2,863,646 12/1958Nelson et a1 55-112 X wherein said distributor flange is provided Withribs which 1,440,887 1/ 1923 Nesbit 55-127 bear against the surface ofthe tube to produce said inclined water channels. FOREIGN PATENTS858,837 12/1962 Germany. References Cited 14,771 6/ 1903 Great Britain.

UNITED STATES PATENTS gf gg- 2,937,709 5/ 1960 De Seversky. 2,351,0896/1944 Abbey. 916,211 8/ 1951 Germany. 2,675,477 4/ 1954 Teslner DENNISE. TALBERT, 111., Primary Examiner 2,168,402 8/ 1939 Fitzgerald 307-1062,000,020 5/ 1935 Heinrich. US. Cl. X.R.

- 895,729 8/1908 cittrell 55-151 21 s3, 74 R; 23 2 R; 5s 129, 137, 138,139, 148, 152, 2195431 4/1940 Shlvely 55-152 154 229 240 413 418 472-204 312- 261-36 '11 2,412,912 12/1946 Schmidt et a1. 55-119 k 2,782,8672/1957 Hall -139 2,888,092 5/1959 Powers 55-155

